It’s hard to imagine that rubbing very hard steel across leather will actually do much of anything to it, but the effect of stropping a razor on an untreated leather strop prior to shaving is evident when you shave. So what’s the deal? What does stropping do to the edge? And the answer is that it burnishes the steel and will actually remove the steel, albeit very slowly and in a very non-agressive fashion. If you take a super clean piece of very hard steel and rub the hell out of a piece of leather, it will blacken and that’s the metal being removed through the burnishing action. Use some very light colored leather and you’ll see the color turning in pretty short order.
The more you strop, the more wear there is on the steel and after some time, what scratch patterns on the edge existed after honing get completely worn away. This is what many notice to be the “sweet spot” in their honing cycle when their razor seems to improve in shaving quality with each subsequent shave for a while before eventually possibly degrading again. Once the edge has been really well burnished to the point where there is as close to a zero radius bevel and as close to zero deviation of the straight line edge, a razor is as smooth and sharp as it’s ever going to be.
You can definitely see the polishing/burnishing effect of stropping on the edge under high magnification. At the micro level, the burnishing wears away the high points of the edge first, removing the tiny “teeth” in a good, gentle fashion and that’s important because the “teeth” or “fin” that we’ve come to think of as the razor’s edge tend to be uneven, brittle and easily broken off. Breaking them off with use is not good because that leaves micro-level flat spots. Worn away through the burnishing action of leather, the flat spots are minimized to the greatest degree and the edge is both stronger and smoother because of that.
Although it was rarely, if ever explained why in old barber’s handbooks, (or today’s instruction) it was always and still is strongly recommended to strop thoroughly after honing before shaving. I’m sure this has puzzled some people who come off a hone with a razor that cuts free standing hair and passes hanging hair tests, etc…. it’s certainly sharp enough to shave right off the hone, so why not? And the answer is because the burnishing effect of the leather makes for not only an even better edge, but for a smoother AND more durable edge. The more even the edge, the less leverage there is for any small microstructures (peaks) to be snapped off in use.
Do NOT put paste or abrasive of any kind on your daily razor strop. You you don’t want to create scratch patterns with your stropping, you want to reduce them and burnishing on leather that has not been coated with any sort of abrasive is just the thing for this.